Inter-observer agreement of Medical Research Council-sum score and handgrip strength in the ICU

Introduction

Muscle weakness often complicates critical illness and is associated with prolonged duration of rehabilitation, increased mortality and limiting functional outcome even years later [1]. To examine the effects of interventions on this complication, reliable measurements of muscle force in critically ill patients are needed. We aimed to examine, in critically ill patients, the inter-observer agreement on the Medical Research Council (MRC)-sum score and handgrip strength, two methods to quantify muscle force.

Methods

All patients were included in an ongoing randomized controlled trial examining two feeding strategies (ClinicalTrials. gov:NCT 00512122) and constituted a cross-sectional, randomly selected sample. Patients were studied on median ICU day 17 (11 to 29.5). Two observers independently measured the MRC-sum score in 75, and handgrip strength in 46 critically ill patients.

Results

The intra-class correlation coefficient (ICC) for the MRC-sum score was 0.95 (0.92 to 0.97), with weighted kappa coefficient for individual muscle group scores of 0.83 ± 0.03. Kappa coefficient was 0.68 ± 0.09 for identifying patients with MRC-sum score <48, 0.88 ± 0.07 for MRC-subtotal in the upper limbs <24, and 0.93 ± 0.07 for MRC-sum score <36. The ICCs for left and right handgrip strength were respectively 0.97 (0.94 to 0.98) and 0.93 (0.86 to 0.97).

Conclusions

We found very good inter-observer agreement, both for MRC-sum score and for handgrip strength in critically ill patients. When applying MRC-sum score <36 as a cut-off for severe weakness, agreement was excellent supporting its use as an outcome parameter for interventional studies. Agreement on identifying significant weakness (MRC-sum <48) was good. For an equivalent cut-off to identify significant weakness in the upper limbs (< 24), agreement was very good. It remains to be determined whether this may be used as a substitute for the total MRC-sum score.